API: Add ability to update_now repository from mirror (pull)

Description

It would be handy to be able to trigger a mirroring via the API as can be done via the web interface and the /:namespace/:project/mirror/update_now endpoint.

Looks like mirroring is coming to CE so I am opening this here.

Proposal

Add a

POST /projects/:id/update_now

endpoint to trigger updating.

Documentation blurb

Trigger an update

Triggers pulling in of changes for repository mirroring. Returns status code 304 if the project does not have a mirror respository.

POST /projects/:id/update_now

Parameters:

Attribute	Type	Required	Description
id	integer/string	yes	The ID or URL-encoded path of the project

curl --request POST --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" "https://gitlab.example.com/api/v4/projects/5/update_now"

Example response:

{
  "id": 3,
  "description": null,
  "default_branch": "master",
  "visibility": "internal",
  "ssh_url_to_repo": "git@example.com:diaspora/diaspora-project-site.git",
  "http_url_to_repo": "http://example.com/diaspora/diaspora-project-site.git",
  "web_url": "http://example.com/diaspora/diaspora-project-site",
  "tag_list": [
    "example",
    "disapora project"
  ],
  "name": "Diaspora Project Site",
  "name_with_namespace": "Diaspora / Diaspora Project Site",
  "path": "diaspora-project-site",
  "path_with_namespace": "diaspora/diaspora-project-site",
  "issues_enabled": true,
  "open_issues_count": 1,
  "merge_requests_enabled": true,
  "jobs_enabled": true,
  "wiki_enabled": true,
  "snippets_enabled": false,
  "container_registry_enabled": false,
  "created_at": "2013-09-30T13:46:02Z",
  "last_activity_at": "2013-09-30T13:46:02Z",
  "creator_id": 3,
  "namespace": {
    "id": 3,
    "name": "Diaspora",
    "path": "diaspora",
    "kind": "group",
    "full_path": "diaspora"
  },
  "import_status": "none",
  "archived": true,
  "avatar_url": "http://example.com/uploads/project/avatar/3/uploads/avatar.png",
  "shared_runners_enabled": true,
  "forks_count": 0,
  "star_count": 1,
  "public_jobs": true,
  "shared_with_groups": [],
  "only_allow_merge_if_pipeline_succeeds": false,
  "only_allow_merge_if_all_discussions_are_resolved": false,
  "request_access_enabled": false
}